1. Field of the Invention
This invention relates to a constant-speed cruising control apparatus for vehicles and, more particularly to an improved and novel constant-speed cruising control apparatus suitable for vehicles mounted with an automatic transmission or torque converters.
2. Description of the Prior Art
Heretofore, in a constant-speed cruising apparatus for the vehicles such as an automobile and the like which are mounted with the automatic transmissions or torque converters, the cruising speed is controlled constantly by actuating the throttle valve in the opening or closing direction using a diaphragm type actuator or a servomotor.
FIG. 2 is a block diagram showing a conventional constant-speed cruising control apparatus for vehicles.
In the illustrated constant-speed cruising control apparatus 100, numeral 10 is a read switch used to detect a running speed of the vehicle and numeral 12 is a permanent magnet to be rotated through a speed meter cable. The reed switch 10 is changed on and off by the rotation of the permanent magnet 12, and generates a pulse signal having a frequency proportional to the actual running speed of the vehicle. The pulse signal is converted at a frequency-voltage conversion circuit (hereinafter, referred to as F/V conversion circuit) 14 into a signal having a voltage proportional to the vehicle speed, that is a vehicle speed signal. The signal output from the F/V conversion circuit 14 is sent to a memory circuit 18 if an analog switch 16 is turned on, and the signal at the time just before switching-off the analog switch 16 is stored in the memory circuit 18 as a cruising speed signal corresponding to the cruising speed to be maintained constantly. The memory circuit 18 is constructed conventionally from a semiconductor memory, and the signal stored in the memory circuit 18 is compared in a comparing circuit 20 with the signal output from the F/V conversion circuit 14, that is the signal corresponding to the actual vehicle speed. The result of the comparison is sent to an actuator 28 through an AND gate 24 and a drive circuit 26, and a control valve 28a of the actuator 28 is driven.
In order to control the drive carried out by the actuator 28, the comparing circuit 20 generates a pulsed output signal of which duty ratio changes so as to decrease the time for applying an electric current to the control valve 28a in case the actual vehicle speed corresponding to the signal output from the F/V conversion circuit 14 is higher than the cruising speed corresponding to the signal stored in the memory circuit 18, and changes so as to increase the time for applying an electric current to it in a case contrary to this.
The AND gate 24 opens only when a signal output from a self-hold circuit 34 is logic "1", and transmits the signal output from the comparing circuit 20 into the drive circuit 26.
A set switch 38 to be operated for setting the cruising speed generates an output signal having logic "0" at the time of being operated by the driver, and generates an output signal having logic "1" when the operation is released. The signal output from the set switch 38 is impressed on the analog switch 16 and the self-hold circuit 34 through an inverter 40. Accordingly, the analog switch 16 is changed on if the set switch 38 is operated, and changed off when the operation is finished. The self-hold circuit 34 is also set when the set switch 38 is operated.
The self-hold circuit 34 consists of a flip-flop, for example, self-holds that the setting operation is carried out by the set switch 38, and transmits the output signal to the drive circuit 36 and works to open the AND gate 24 during the self-holding. The self-hold circuit 34 is reset when one of a stop lamp switch 44, a parking brake switch 46 and a neutral start switch 48 is changed on, and makes the drive circuit 36 off and works to close the AND gate 24. The stop lamp switch 44, the parking brake switch 46 and the neutral start switch 48 are structured as to be changed on when a brake pedal is depressed, a parking brake is operated and the automatic gear is shifted into the neutral position, respectively.
The actuator 28 is provided with the control valve 28a, a cancel valve 28b and so on. The control valve 28a is controlled so as to be opened or closed according to the signal output from the AND gate 24 through the drive circuit 26. Namely, when the self-hold circuit 34 is set and the drive circuit 26 is applied with an electric current, the control valve 28a isolates the external atmosphere to be introduced through a port 28c and introduces negative pressure of the intake manifold into a negative pressure chamber 28e through a port 28d, and the control valve 28a isolates the negative pressure through the port 28d and introduces the atmospheric pressure into the negative pressure chamber 28e through the port 28c if the electric current is shut off to the drive circuit 26. The ratio between the atmospheric pressure introduced through the port 28c and the negative pressure of the intake manifold introduced through the port 28d is controlled according to the duty ratio of the pulse signal output from the comparing circuit 20. The cancel valve 28b is controlled so as to be opened or closed according to the signal output from the self-hold circuit 34 through the drive circuit 36. Namely, when the self-hold circuit 34 is set and the drive circuit 36 is applied with an electric current, the cancel valve 28b isolates the external atmosphere through a port 28f, and if the self-hold circuit 34 is reset and the electric current is shut off to the drive circuit 36, the cancel valve 28b introduces the atomospheric pressure into the negative pressure chamber 28e. Therefore, a diaphragm 28g moves by controlling the pressure in the negative pressure chamber 28e, thereby driving a rod 28h connected with an accel linkage (not shown) in the axial direction and controlling the opening of the throttle valve, consequently the vehicle is maintained in the state of constant-speed cruising.
The vehicle mounted with the conventional constant-speed cruising control apparatus described above sometimes becomes incapable of maintaining the setting cruising speed in a case of running on a steep uphill road. Namely, the constant-speed cruising control apparatus having the actuator of the diaphragm type as shown in FIG. 2 has a weakpoint that it is impossible to run at the constant cruising speed because the apparatus becomes incapable of following the setting cruising speed when the vehicle travels on the steep uphill road. In other words, because load applied on the vehicle increases as the rising gradient of the road becomes steep, whereby the rotational frequency of the engine decreases and the negative pressure of the intake manifold becomes insufficient as natural results in the case of using the actuator of the diaphragm type, the actuator is not capable of opening the throttle valve sufficiently and it is not possible to control the vehicle so as to run at the constant cruising speed.
Also in case of using the servomotor for actuating the throttle valve, it is impossible to follow driving torque against the rising gradient of the road and the tractive force of the engine becomes insufficient in a case in which reduction ratio of the transmission of the vehicle is not so high even if the throttle valve can be opened sufficiently, there is a weakpoint in that the actual vehicle speed decreases as compared with the setting cruising speed.
A constant-speed cruising control apparatus is disclosed in U.S. Pat. No. 4,421,192, which controls the vehicle so as to maintain the constant-speed cruising by shifting the transmission down if the apparatus is unable to follow the setting cruising speed. Namely, the apparatus disclosed in the Japanese Patent Document is so designed as to restore the vehicle speed by shifting automatically the transmission to the third gear from the top gear, for example, when the actual vehicle speed decreases as compared with the setting cruising speed by the order of 7 km/h.
However, in the above-mentioned constant-speed cruising control apparatus which shifts the automatic transmission down automatically, the automatic transmission is not shifted down until the actual vehicle speed becomes lower than the setting cruising speed by 7 km/h even when the actual speed of the vehicle drops gradually on the uphill road, therefore there is a problem in that it is not possible to perform the constant-speed cruising control speedily.